There are 4 hidden characters in Super Smash Brothers. They are Luigi, Ness,
Captain Falcon, and Jigglypuff. Here is how to unlock them.
Luigi:
finish bonus 1 practice with all of the initially available characters, then
defeat him to unlock him.
Jigglypuff:
Finish 1-player mode with any character.
Captain Falcon:
Finish 1-player mode in under 5 minutes.
Ness:
This one's a toughie. You have to finish 1-player mode on normal difficulty
with 3 lives and no continues. then defeat him.
Mushroom Kingdom stage:
Finish 1-player mode with all 8 initially available characters.
Careful the pirahna plants bite! But the really cool thing is that you can go
down the warp pipes!!
Item Switch:
I' actually not sure, but I think you have to play a certain number of VS.
matches. How many, i don't know. Just keep playing VS. matches until you see a
message telling you that you can access the Item Switch.
There are probably more secrets to be found, I just havent found them yet.